Polybutadiene has a so-called microstructure in which a binding segment formed by polymerization at the 1,4-positions (1,4-structure) and a binding segment formed by polymerization at the 1,2-positions (1,2-structure) are copresent in the molecular chain. The 1,4-structure is further classified into a cis-structure and a trans-structure. On the other hand, the 1,2-strucutre takes a structure containing a vinyl group as a side chain.
Hitherto, a production process of a vinyl-cis-polybutadiene rubber composition has been carried out in an aromatic hydrocarbon-based solvent such as benzene, toluene, and xylene. When such a solvent is used, since the viscosity of a polymerization solution is high, there were problems in stirring, heat transfer, delivery, and so on, and excessive energy was required for recovering the solvent.
As the foregoing production process, there is known a process in which 1,3-butadiene is subjected to cis-1,4-polymerization in the foregoing inert organic solvent by using a catalyst obtainable from water, a soluble cobalt compound and an organoaluminum halide represented by the general formula, AlRnX3-n (wherein R represents an alkyl group having from 1 to 6 carbon atoms, a phenyl group, or a cycloalkyl group; X represents a halogen atom; and n represents a numeral of from 1.5 to 2) to produce BR, and 1,3-butadiene is then subjected to syndiotactic 1,2-polymerization (hereinafter abbreviated as “1,2-polymerization”) in the presence of a catalyst obtainable from a soluble cobalt compound, an organoaluminum compound represented by the general formula, AlR3 (wherein R represents an alkyl group having from 1 to 6 carbon atoms, a phenyl group or a cycloalkyl group) and carbon disulfide by adding or not adding 1,3-butadiene and/or the foregoing solvent to this polymerization system (see, for example, JP-B-49-17666 (Patent Document 1) and JP-B-49-17667 (Patent Document 2)).
Furthermore, for example, JP-B-62-171 (Patent Document 3), JP-B-63-36324 (Patent Document 4), JP-B-2-37927 (Patent Document 5), JP-B-2-38081 (Patent Document 6) and JP-B-3-63566 (Patent Document 7) describe a process in which the production is carried out by subjecting 1,3-butadiene to cis-1,4-polymerization in the presence or absence of carbon disulfide, or after the production, 1,3-butadiene and carbon disulfide are separated and recovered, thereby circulating 1,3-butadiene not substantially containing carbon disulfide or the foregoing inert organic solvent. In addition, JP-B-4-48815 (Patent Document 8) describes a rubber composition having excellent tensile stress and flex crack growth resistance, in which a compounded material thereof has a small die swell ratio and a vulcanizate thereof is suitable as a sidewall of tire.
Furthermore, JP-A-2000-44633 (Patent Document 9) describes a process in which the production is carried out in an inert organic solvent containing, as the major component, a C4 fraction such as n-butane, cis-2-butene, trans-2-butene, and butene-1.It is described that in this process, 1,2-polybutadiene contained in the rubber composition is a short fiber crystal, and the distribution of the major axis length of the short fiber crystal is such that 98% or more of the fiber length is less than 0.6 μm and 70% or more thereof is less than 0.2 μm. Also, it is described that in the resulting rubber composition, the moldability, tensile stress, tensile strength and flex crack growth resistance of a cis-1,4-polybutadiene rubber (hereinafter abbreviated as “BR”) are improved. However, rubber compositions in which various characteristics are improved were demanded depending upon the utility.
Patent Document 1: JP-B-49-17666
Patent Document 2: JP-B-49-17667
Patent Document 3: JP-B-62-171
Patent Document 4: JP-B-63-36324
Patent Document 5: JP-B-2-37927
Patent Document 6: JP-B-2-38081
Patent Document 7: JP-B-3-63566
Patent Document 8: JP-B-4-48815
Patent Document 9: JP-A-2000-44633
In general, with respect to the performance of automobile tire, there are required excellent wet skid properties as a braking characteristic and excellent rolling resistance (tan δ) or abrasion resistance as a fuel consumption characteristic. However, it is known that these characteristics are in an antinomy relation with each other. In recent years, though there has been made a proposal to compound silica having excellent wet skid properties, thereby highly balancing the foregoing characteristics, such is not satisfactory yet. It is known that though the silica compounding is excellent in the wet skid properties and fuel consumption properties, the abrasion resistance and processability are lowered. While the abrasion resistance is improved by the use of high cis-BR, there is some possibility that the wet skid properties are lowered, and therefore, improvements were demanded.
Furthermore, in general, it is known that a proportion of the tire rolling resistance occupying in the running resistance which largely affects the fuel consumption of an automobile is large and that the affect due to an energy loss of sidewall rubber is relatively large. For that reason, rubbers having a low dissipation factor such as natural rubbers, isoprene rubbers, butadiene rubbers, and mixtures thereof have hitherto been used in the sidewall part. However, it is required that the rolling resistance of tire is further improved.
As a method for reducing the dissipation factor of the sidewall rubber, the use of carbon black having low reinforcing properties or a reduction of the compounding amount of carbon black may be thought. However, there is encountered such a problem that the die swell at the time of extrusion processing becomes large, and therefore, it is difficult to make the thickness of the sidewall member thin or to realize an enhancement of uniformity of tire. For that reason, there is demanded a method for realizing low fuel consumption while keeping a small die swell with respect to an extrudate thereof.
Furthermore, in general, tires are required to have excellent drivability and durability and so on. In particular, from the standpoint of safety, the tires are required to have excellent wet skid resistance on a wet road surface. Moreover, on the basis of social requirements for realizing conservation of natural resources in recent years, tires having a low rolling resistance, namely tires with a low energy loss, are being researched and developed. While the energy loss of tire as consumed by free rotation varies depending upon the tire structure, etc., about a half of the whole energy is consumed in the tread part. Accordingly, if the energy loss of the tread rubber is reduced, a tire with a low energy loss at the time of rolling is obtained.
Then, it is attempted to modify the tread rubber such that the energy loss is low. However, such modification of the rubber tends to lower the wet skid properties. Since an improvement of the rolling resistance and an improvement of the wet skid properties generally contradict to each other, various designs for the improvements in the tire structure are attempted in order to make them compatible with each other. As one of the designs, there is enumerated a method for forming a tread into two layers consisting of a cap tread and a base tread. That is, this method is to form the tread into two layers of a cap tread with excellent wet skid properties and a base tread with a low energy loss, thereby not only enhancing the wet skid properties of tire as a whole but also lowering the energy loss.
The rubber for cap tread is required to have a high elastic modulus and molding stability derived from the abrasion resistance and high-speed running properties in addition to the wet skid properties. As a method for obtaining a rubber with a high elastic modulus, various methods have hitherto been attempted. The method for compounding a large amount of carbon black is not preferable because unification of rubbers is poor in the processing step, an electrical power load increases at the time of kneading or extrusion, and ML of the compounded material becomes large, whereby difficulty is accompanied at the time of extrusion molding. The method for compounding a large amount of sulfur involves such defects that sulfur causes blooming and that the crack growth becomes fast due to an increase of the degree of crosslinking. With respect to the addition of a thermosetting resin, since the thermosetting resin is low in compatibility with usually used natural rubbers or diene-based rubbers, when a large amount of the thermosetting resin is compounded, satisfactory dispersion is hardly obtained. Furthermore, since this mixed compound is hard even in an unvulcanized state, the load may become large in kneading or extrusion, or the molding processability of tire may be deteriorated. In a method for merely blending and compounding single fibers, since binding between short fibers and the rubber is insufficient, creep may become large, or fatigue life may be lowered.
As the rubber for base tread, a rubber with low exothermic characteristics is required. Examples of rubbers with low exothermic characteristics include natural rubbers, isoprene rubbers, and cis-1,4-polybutadiene rubbers, and compounded materials of such a rubber with carbon black are used. In order to reveal low exothermic characteristics by other materials than rubbers, it may be thought to use carbon black having a large particle size and low reinforcing properties or to lower the compounding amount of carbon black. However, these methods involve such problems that the elastic modulus or fatigue resistance of rubber is lowered and that the die swell becomes large. Furthermore, there is a method for revealing low exothermic characteristics by compounding short fibers of nylon or vinylon, thereby making the rubber have a high elastic modulus. However, since these short fibers are insufficient in adhesion to rubbers, there is encountered a problem that the fatigue life is short.
On the other hand, when a cap/base system is employed in passenger automobile tires, at the time of co-extrusion of a cap tread and a base tread, there is caused a problem that an extrudate is warped because of a difference in die swell between the both members. If the die swell of the base tread rubber is made small, this problem is liable to be solved. If a large amount of carbon black with high reinforcing properties is compounded, though the die swell become small, the heat generation becomes large. Thus, a method which is satisfied with the die swell and low heat generation at the same time is desired.
Furthermore, as rubber members to be disposed in the surroundings of tire bead, high-hardness rubbers are used. Examples thereof include a rubber composition having an increased compounding amount of carbon black and a rubber composition having a novolak-type phenol resin compounded therein (see JP-B-57-30856) and a rubber composition having short fibers and an olefin-based resin compounded therein (see JP-A-7-315014).
With respect to a method for obtaining a high-hardness rubber composition, it is general to increase carbon black or to compound a fiber, a resin, etc. However, in all of these methods, there may be the case where the heat generation at the time of repeated deformation becomes large, whereby the durability and fatigue resistance are lowered. Thus, improvements are required.
Furthermore, in general, in radial tires, a steel cord is used in view of high-speed durability and high-speed drivability. In the case of using a steel cord, a very large strain is liable to be gathered in a rubber in the vicinity of the steel cord at the time of running of tire. Accordingly, the rubber for steel cord is required to have a high elastic modulus and excellent adhesiveness to metals. Even in radial tires or bias tires using an organic fiber cord, rubbers having a high elastic modulus are preferable as the rubber for cord from the viewpoint of durability.
As a method for obtaining a rubber with a high elastic modulus, various methods have hitherto been attempted. The method for compounding a large amount of carbon black is not preferable because unification of rubbers is poor in the processing step, an electrical power load increases at the time of kneading or extrusion, and ML of the compounded material becomes large, whereby difficulty is accompanied at the time of extrusion molding. The method for compounding a large amount of sulfur involves such defects that sulfur causes blooming and that the crack growth becomes fast due to an increase of the degree of crosslinking. With respect to the addition of a thermosetting resin, since the thermosetting resin is low in compatibility with natural rubbers or diene-based rubbers as usually used as a cord coating rubber, dispersion failure likely occurs, and crack resistance is deteriorated. Furthermore, conventionally known rubber compositions for tire cord coating are small in green strength. Thus, materials having a large green strength are required in view of molding processability.